Wetting, Adsorption, and Cleaning Processes - Surface and Colloid Chemistry: Principles and Applications - page 138

Energy is needed to break such a contact, more so with some joined items than others.
If we consider the adhesion of plastic on glass, the highest adhesion will be obtained
if the adhesive fills all the valleys and crevices of each adhered body surface. This
will remove any air pockets that do not contribute to adhesion. The role of the adhe-
sive or glue is to provide mechanical interlocking of the adhesive molecules.
The strength of the bond is dependent upon the quality of this interlocking inter-
face. For achieving optimum bonding, chemical or physical abrading is used. The
abrading process creates many useful properties at the solid surface: enhancement
of mechanical interlocking, creation of a clean surface, formation of a chemically
reactive surface, and an increase in surface rate (a smooth surface has lower surface
area than a rough surface). Diffusion bonding is a form of mechanical interlocking
that occurs at the molecular level in polymers.
The science behind bonding technology is very extensive. A brief description
along with some examples is given in the following text. It is important to prime the
surfaces of the layers to be bonded, that is, cover the surfaces with a dilute solution
of the adhesive mixed with an organic solvent to obtain a dried film of thickness
between 0.0015 and 0.005 mm.
Another example is concerned with epoxy adhesives. Epoxies are best, but an
alloy such as epoxy-phenolic or epoxy-polysulfone may offer improved peeling
resistance.
Dilute the adhesive until it has a lower surface tension than either of the bonding
surfaces. The surface tensions can be compared by using a wetting test (i.e., by wet-
ting the surface with the adhesive and measuring the contact angle). A low contact
angle (<90) indicates good wetting and an appropriate adhesive.
Many theories have been developed to explain the process of bonding in adhesive
structures. According to the mechanical bonding theory, an adhesive needs to fill the
valleys and crevices of each adherend (body to be bonded) and displace trapped air
to work effectively. Adhesion is the mechanical interlocking of the adhesive and the
adherend together, and the overall strength of the bond is dependent upon the quality
of this interlocking interface. This can be chemical or physical abrading for optimum
bonding. Abrading the adherend does the following:
1. Enhances mechanical interlocking.
2. Creates a clean corrosion-free surface.
3. Forms a chemically reactive surface.
4. Increases the bonding surface area.
Diffusion bonding is a form of mechanical interlocking that occurs at the molecu-
lar level in polymers. The adsorption mechanism theory suggests that bonding
is the process of intermolecular attraction (van der Waals bonding or permanent
dipole, for example) between the adhesive and the adherend at the interface. An
important factor in the strength of the bond, according to this theory, is the wetting
of the adherend by the adhesive. Wetting is the process in which a liquid spreads
onto a solid surface and is controlled by the surface energy of the liquid-solid
interface versus the liquid-vapor and the solid-vapor interfaces. In a practical